Suggest std::iter::{once, empty}

[matthiasbeyer]

I don't know whether clippy is capable of doing this, but it would be nice to suggest std::iter::once() and std::iter::empty(), especially when things like vec![element].iter() (or similar) is used.

This is possible but the type of std::iter::once() \ std::iter::empty() isn't the same as vec![..].iter(). This means that the lint needs to be careful when it makes the suggestion. (See #3307 for an example of what can go wrong.) It's probably only safe when the result is immediately used by another iterator method.

[jfrimmel]

One thing where this lint could also be applied is this pattern:

Some(x).into_iter(); // -> std::iter::once(x)
None.into_iter(); // -> std::iter::empty()

I see this relatively common, but the expressed meaning of these statements isn't as clear as it could be if the standard iterator functions would be used.

I think that the statement of @mikerite is also true in this case:

It's probably only safe when the result is immediately used by another iterator method.

[llogiq]

If returning it as impl Iterator, it would also be OK to change, I guess.

[jplatte]

Just saw Some(foo).into_iter().chain(something_else).collect() today. So in addition to the possibility of returning impl Iterator, another (maybe more likely) scenario where this lint could be applied is when the iterator (chain) is consumed in the same expression, though I'm not sure how hard that is to detect.